1. Field of the Invention
The present invention relates to a receive method and a receiver in a communication system. More particularly, the present invention relates to a receive method and a receiver in a communication system which converts a signal into a carrier band for transmission in which desired signal components are extracted in baseband.
2. Description of the Related Art
If it becomes possible that signals of various systems and various frequencies are received by one receiver, various information can be obtained by one terminal. However, as for a wireless communication system, an information signal is converted into a carrier having a frequency ranging from several hundreds MHz to several GHz for transmission. In this case, different frequencies are assigned to wireless communication systems such that the frequencies for each wireless communication system do not overlap one another since communication is performed in a medium, that is, one free space. Then, the information is transmitted by the carrier having an assigned frequency. Therefore, a frequency band includes a lot of systems so that the systems are placed densely on the frequency axis. Thus, a filter which is adapted to a channel band of the receiver and has high selectivity becomes necessary in order to extract a desired frequency signal.
FIG. 1 shows a configuration of a receiver in a conventional wireless communication system. More particularly, FIG. 1 shows several parts of a double super heterodyne receiver which is used in an analog car telephone system of NTT. The receiver shown in FIG. 1 includes an antenna 1, a band-pass filter (BPF) 2 of an RF (Radio Frequency) band which is a carrier band of a first stage, a local oscillator 3 for converting a signal to a first IF (Intermediate Frequency) band, a multiplier 4, a band-pass filter for removing higher harmonic components included in the output from the multiplier 4, a local oscillator 6 for converting a signal to a second IF (Intermediate Frequency) band, a multiplier 7, a narrow band-pass filter 8 for removing higher harmonic components included in an output from the multiplier 6 and for selecting a self channel, an amplifier 9 for absorbing receive power variation associated with movement of a terminal, a band-pass filter 10 for separating a digital signal which is sent as a control signal and a voice signal, a demodulator 11 and an output terminal 12.
According to the configuration shown in FIG. 1, the band-pass filter 8 has high selectivity and extracts only a signal of the self channel. However, in the configuration, in order to demodulate signals of various systems, it is necessary to change band of the band-pass filters 2, 5, especially 8 according to the signal of the system. However, generally, it is difficult to change frequency band characteristics of an analog filter of RF/IF bands. That is, for conforming to systems which have various frequency bands, it is impossible to select a desired band signal by the RF band-pass filter or the IF band-pass filter.
Generally, signal bands differs according to systems. Therefore, in order to receive signals of various systems, it is necessary to provide an RF/IF filter which has a conceivable maximum signal band for systems to be received. In this case, it becomes possible to receive signals of various systems by using a base-band filter which easily realizes changeability of frequency band characteristics and high selectivity for selecting desired signal, where the base-band filter may be a filter realized by digital signal processing.
When frequency conversion is performed, a frequency synthesizer is necessary. When assuming that systems including a very narrow bandwidth system are used, the frequency synthesizer needs to be highly accurate and stable over a wide frequency range. In addition, for the frequency synthesizer to select a frequency band freely, the circuit of the frequency synthesizer becomes complex. Thus, there occurs a problem in that the frequency synthesizer can not be used for a system like a mobile communication system which requires low power consumption for devices. Therefore, a signal is covered into an appropriate IF frequency band and is converted into a digital signal by an analog/digital converter temporarily. After that, the ranges of systems with which communication can be performed can be expanded by predicting the carrier frequency accurately and demodulating by using a high capability digital signal processing technique. In addition, according to this configuration, since the frequency is converted only to the IF frequency band, it becomes possible to avoid characteristic degradation due to DC (Direct Current) components in the receiver which occurs when the frequency is converted to the baseband.
However, when the receiver is configured such that it includes (α) an analog wideband band-pass filter and a channel filter operated by digital signal processing and (β) analog/digital conversion in IF frequency bands, there is a problem in that SNR (Signal to Noise Ratio) degrades since image frequency band components are mixed in signal components due to relationship between IF frequency band and band of the analog band-pass filter.
FIGS. 2A and 2B show relationship between the image frequency band components and the bandwidth of the band-pass filter. In principle, the digital wireless communication system converts only real frequency band components to a carrier band f+Δf when performing communication. At this time, in the receiver, when the signal is converted by the local oscillation frequency f so that signal of IF frequency band Δf is generated, the signal of carrier band f−Δf is also converted to the IF frequency band Δf as an interference wave at the same time in principle. This is a cause of degradation of SNR. For example, when phase modulation is used for both of the desired signal and the interference signal, the interference signal appears in the IF frequency band as shown in the following equation (1),                                           LPF            1                    ⁡                      [                                          {                                                      A                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                                          (                                                                                                    (                                                                                          ω                                c                                                            +                                                              Δ                                ⁢                                                                                                                                  ⁢                                ω                                                                                      )                                                    ⁢                          t                                                +                                                  a                          k                                                                    )                                                        +                                      B                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                                          (                                                                                                    (                                                                                          ω                                c                                                            -                                                              Δ                                ⁢                                                                                                                                  ⁢                                ω                                                                                      )                                                    ⁢                          t                                                +                                                  b                          k                                                                    )                                                                      }                            *              cos              ⁢                                                          ⁢                              (                                                      ω                    c                                    ⁢                  t                                )                                      ]                          =                                            A              2                        ⁢            cos            ⁢                                                  ⁢                          (                                                Δω                  k                                +                                  a                  k                                            )                                +                                    B              2                        ⁢            cos            ⁢                                                  ⁢                          (                                                Δω                  ⁢                                                                          ⁢                  t                                -                                  b                  k                                            )                                                          (        1        )            wherein ω indicates each frequency, ω=2πf, t indicates time variable, LPF1 indicates a function for eliminating high-frequency band components, ak and bk indicate information components of the desired signal and the interference signal, A and B indicate levels of the desired signal and the interference signal. Conventionally, to avoid this problem, as shown in FIG. 2A, a band-pass filter for suppressing the signal of the carrier band f−Δf is placed before the frequency converter. However, when bandwidth of the band-pass filter is widened in order to receive various frequency bands of various systems, the signal of the carrier band f−Δf is converted to the IF frequency band.
To overcome this problem, an image frequency canceler is proposed in which the image frequency band components are removed after orthogonal quasi-coherent detection is performed on the signal of RF band. A configuration of the image frequency canceler is shown in FIG. 3. The image frequency canceler includes an antenna 13, a first stage band-pass filter 14, branch circuits 15 and 19, multipliers 16 and 17, a π/2 phase shifter 18, low-pass filters 21, 22, 27 and 28, analog/digital converters 23 and 24, a complex frequency converter 25 performing multiplication of analytic sine wave exp(−jΔωkT) of IF frequency band, output terminals 29 and 30, wherein T indicates a sampling frequency. In the example shown in FIG. 3, orthogonal quasi-coherent detection is performed on the real frequency signal of the carrier band. That is, the following quadrature component is generated in addition to the interference signal shown in (1).                                           LPF            1                    ⁡                      [                                          {                                                      A                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                                          (                                                                                                    (                                                                                          ω                                c                                                            +                              Δω                                                        )                                                    ⁢                          t                                                +                                                  a                          k                                                                    )                                                        +                                      B                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                                          (                                                                                                    (                                                                                          ω                                c                                                            -                                                              Δ                                ⁢                                                                                                                                  ⁢                                ω                                                                                      )                                                    ⁢                          t                                                +                                                  b                          k                                                                    )                                                                      }                            *              sin              ⁢                                                          ⁢                              (                                                      ω                    c                                    ⁢                  t                                )                                      ]                          =                                            A              2                        ⁢                          sin              ⁡                              (                                                      Δ                    ⁢                                                                                  ⁢                                          ω                      k                                                        +                                      a                    k                                                  )                                              +                                    B              2                        ⁢                          sin              ⁡                              (                                                      Δω                    ⁢                                                                                  ⁢                    t                                    -                                      b                    k                                                  )                                                                        (        2        )            
When the above-mentioned complex multiplication is performed for the equation (1) and (2) and high-frequency band components appearing in ±2Δω is removed, the following desired signal can be obtained,                                                         LPF              2                        ⁡                          [                                                {                                                                                    A                        2                                            ⁢                                              cos                        ⁡                                                  (                                                                                    Δω                              ⁢                                                                                                                          ⁢                              t                                                        +                                                          a                              k                                                                                )                                                                                      +                                                                  B                        2                                            ⁢                      cos                      ⁢                                                                                          ⁢                                              (                                                                                                            -                              Δω                                                        ⁢                                                                                                                  ⁢                            t                                                    +                                                      b                            k                                                                          )                                                                              }                                ⁢                cos                ⁢                                                                  ⁢                                  (                                      Δω                    ⁢                                                                                  ⁢                    t                                    )                                            ]                                -                                    LPF              2                        ⁡                          [                                                {                                                                                    -                                                  A                          2                                                                    ⁢                                              sin                        ⁡                                                  (                                                                                    Δω                              ⁢                                                                                                                          ⁢                              t                                                        +                                                          a                              k                                                                                )                                                                                      +                                                                  B                        2                                            ⁢                      sin                      ⁢                                                                                          ⁢                                              (                                                                              Δω                            ⁢                                                                                                                  ⁢                            t                                                    -                                                      b                            k                                                                          )                                                                              }                                ⁢                sin                ⁢                                                                  ⁢                                  (                                      Δω                    ⁢                                                                                  ⁢                    t                                    )                                            ]                                      =                                            {                                                                    A                    4                                    ⁢                  cos                  ⁢                                                                          ⁢                                      (                                          a                      k                                        )                                                  +                                                      B                    4                                    ⁢                  cos                  ⁢                                                                          ⁢                                      (                                          b                      k                                        )                                                              }                        -                          {                                                                    -                                          A                      4                                                        ⁢                                      cos                    ⁡                                          (                                              a                        k                                            )                                                                      +                                                      B                    4                                    ⁢                                      cos                    ⁡                                          (                                              b                        k                                            )                                                                                  }                                =                                    A              2                        ⁢            cos            ⁢                                                  ⁢                          (                              a                k                            )                                                          (3.1)                                                                    LPF              2                        ⁡                          [                                                {                                                                                    A                        2                                            ⁢                                              cos                        ⁡                                                  (                                                                                    Δω                              ⁢                                                                                                                          ⁢                              t                                                        +                                                          a                              k                                                                                )                                                                                      +                                                                  B                        2                                            ⁢                      cos                      ⁢                                                                                          ⁢                                              (                                                                                                            -                              Δω                                                        ⁢                                                                                                                  ⁢                            t                                                    +                                                      b                            k                                                                          )                                                                              }                                ⁢                sin                ⁢                                                                  ⁢                                  (                                      Δω                    ⁢                                                                                  ⁢                    t                                    )                                            ]                                +                                    LPF              2                        ⁡                          [                                                {                                                                                    -                                                  A                          2                                                                    ⁢                                              sin                        ⁡                                                  (                                                                                    Δω                              ⁢                                                                                                                          ⁢                              t                                                        +                                                          a                              k                                                                                )                                                                                      +                                                                  B                        2                                            ⁢                      sin                      ⁢                                                                                          ⁢                                              (                                                                              Δω                            ⁢                                                                                                                  ⁢                            t                                                    -                                                      b                            k                                                                          )                                                                              }                                ⁢                cos                ⁢                                                                  ⁢                                  (                                      Δω                    ⁢                                                                                  ⁢                    t                                    )                                            ]                                      =                                            {                                                                    -                                          A                      4                                                        ⁢                  sin                  ⁢                                                                          ⁢                                      (                                          a                      k                                        )                                                  +                                                      B                    4                                    ⁢                  sin                  ⁢                                                                          ⁢                                      (                                          b                      k                                        )                                                              }                        +                          {                                                                    -                                          A                      4                                                        ⁢                                      sin                    ⁡                                          (                                              a                        k                                            )                                                                      -                                                      B                    4                                    ⁢                                      sin                    ⁡                                          (                                              b                        k                                            )                                                                                  }                                =                                    -                              A                2                                      ⁢            sin            ⁢                                                  ⁢                          (                              a                k                            )                                                          (3.2)            wherein LPF2 is a function for removing the high-frequency band components appearing in ±2Δω. If the equations (3.1) and (3.2) can be realized perfectly, the image frequency band components can be canceled theoretically. However, in actuality, there is orthogonality error and gain imbalance in the analog quasi-coherent detector. As a result, signal of the carrier band f−Δf is mixed in the equations (3.1) and (3.2) so that SNR is degraded. According to a current analog technique, it is very difficult to realize orthogonality and gain balance in high accuracy in the analog orthogonal quasi-coherent detection for the equations (1) and (2). In reality, adjustment is performed by hand. However, only 20˜30 dB is obtained, which is far from requirement (80˜90 dB in PDC system for example) of the wireless communication system.
In addition, in order to receive signals for various systems, it is necessary to keep orthogonality for signals of various frequencies. However, it is impossible to keep characteristics of the analog π/2 phase shifter over wide band in principle. Therefore, according to the configuration shown in FIG. 3, there is a problem in that enough image frequency remove performance can not be obtained due to orthogonality error and gain imbalance of the analog quasi-coherent detector.
Another method is proposed for avoiding mixing of the image frequency band components in which Δω is set to be far larger than the band of the band-pass filter. In this case, it is necessary to input this high IF signal to the analog/digital converter directly and convert to digital signal. In this case, even if the operation speed of the analog/digital converter is much lower than the IF frequency, signal demodulation is possible when the operation speed is more than four times of the Nyquist rate at the minimum.
Therefore, by utilizing this band-pass flittering technique, the above-mentioned condition (α) and (β) can be satisfied without receiving interference from the image frequency band.
However, in this case, characteristic is seriously degraded due to jitter of sampling clock of the analog/digital converter. Since the amount of degradation is proportional to the IF frequency, it is difficult to use this method for systems having high IF frequency band. As a result, this method can not be used.
Therefore, when the band of the band-pass filter of carrier band is widened in order to receive signals of various systems, interference due to orthogonality disorder becomes serious problem. Therefor, when receiving signals of various systems, there is a method in which RF/IF circuits for receiving each system are provided and are switched appropriately. However, according to this configuration, the number of RF/IF devices increases so that the size of circuits becomes large. As a result, the device size is increased so that the cost is increased. In a mobile communication system in which low cost and miniaturization are required, increase of device size and cost becomes large problem. In addition, when hardware is manufactured, it becomes impossible to remove unnecessary systems. Therefore, in order to introduce a new system, it is necessary to develop hardware from scratch. Therefore, there is a problem in that development cost remarkably increases.
As mentioned above, conventionally, there is a problem in that image frequency band components can not be fully suppressed in the configuration which includes the wide band band-pass filter, the analog orthogonal quasi-coherent detector for frequency conversion, and removes the image frequency band components by using digital complex frequency conversion and filtering. Even if a man performs adjustment for a frequency band by allowing SNR degradation to some extent, there is a problem in that it is impossible to change receive system dynamically when frequency band is changed.
In addition, when the receiver receives signals by using the same hardware for a plurality of systems each placed in its specific frequency, the receiver can not have a filter in which the bandwidth is smaller than the maximum bandwidth in the systems. On the other hand, when considering miniaturization of the receiver, it is effective to perform sampling by IF band and to configure the band-pass filter by a digital filter. At this time, there is a problem in that interference wave is mixed in the desired wave since the minus IF frequency band components are not fully decreased by the RF/IF band.